In a disk storage device such as a magnetic disk device, a continuous recording medium has been in use as the disk storage medium. In recent years, with the purpose of enhancing the recording density, a technique of using a non-continuous medium having recording dots formed thereon has been disclosed as a substitute to the continuous recording medium. This technique is known as bit-patterned media (BPM) recording.
For a magnetic head to perform recording in the isolated storage dots, a writing cycle method is employed. FIG. 1 is an explanatory diagram for explaining the writing cycle method. As illustrated in FIG. 1, the writing cycle method comprises correcting a write clock WRC in accordance with the spacing between recording dots DT and then recording the write data in synchronization with the write clock WRC. That enables writing of data in each dot DT in a correct manner.
In connection with the writing cycle method, an error correction method becomes necessary for correcting errors at the time of writing. Particularly, an error correction method becomes necessary for correcting insertion/deletion errors.
The possible causes of an insertion/deletion error are eccentricity of the disk or high-order variation attributed to processes. FIG. 2 is an explanatory diagram for explaining a disk having eccentricity. As illustrated in FIG. 2, the center of a disk 1000 is out of alignment with respect to a rotation axis 1100. Hence, on the disk 1000, a high-velocity area 1020 and a low-velocity area 1010 come into existence. Because of such differences in the rotating velocity, on a single track as illustrated in FIG. 3, the write clock WRC and the recording dots DT fall out of synchronization.
As illustrated in FIG. 3, in the area of high rotating velocity, the position of each dot DT appears to have widened with reference to the write clock WRC. In such an area, data (single bit) that is in synchronization with the write clock WRC cannot be recorded thereby causing the loss of a single bit. This phenomenon is known as a deletion error.
In contrast, in the area of slow rotating velocity, the position of each dot DT appears to have narrowed with reference to the write clock WRC. At such an area, two recording dots DT get accommodated in synchronization with the write clock WRC thereby leading to the addition of a single bit. This phenomenon is known as an insertion error.
Upon occurrence of a single-bit insertion/deletion error, the recording data subsequent to the location of that error gets shifted thereby triggering a shift error, which in turn leads to a burst error state. For example, in FIG. 4 is illustrated the condition when a single-bit insertion error occurs at the fourth bit of otherwise error-free sector data. As illustrated in FIG. 4, upon occurrence of the single-bit insertion error, the bit sequence subsequent to the location of that error gets shifted to the right thereby leading to a burst error state.
Such a state goes beyond the error correcting capability of an error correcting code such as the Reed-Solomon code that is used in magnetic disk devices. Hence, error correction cannot be accomplished.
A method for correcting insertion/deletion errors in connection with the write synchronization has been proposed for a magnetic tape or an optical disk. For example, as an insertion/deletion error correction method, a 3-bit marker method has been proposed by F. F. Sellers (see F. F. Sellers Jr, “Bitloss and gain correction code,” IRE Trans. Inform. Theory, vol. IT-8, pp. 35-38, January 1962).
FIG. 5 illustrates a 3-bit marker coding method proposed in the conventional technology. As illustrated in FIG. 5, coding is performed by inserting a 3-bit marker MK having the value “100” once after every predetermined bit length (herein, m>1 bit) in an original data string.
Upon occurrence of an insertion/deletion error, the value of the 3-bit marker MK gets shifted due to the occurrence of a shift error. Thus, correction of the insertion/deletion error is performed by referring to the shifted value.
Meanwhile, a method of using a 2-bit marker has been disclosed (for example, see Japanese Patent Application Publication (KOKAI) No. H7-176139) as a substitute to the use of a 3-bit marker method. In the 2-bit marker method, coding is performed by inserting a 2-bit marker MK having the value “01” once after every predetermined length, followed by the detection of an insertion/deletion error.
In order to enhance the recording density, the redundant bits need to be reduced. In the case of employing a marker method, the 2-bit marker method produces less redundancy than the 3-bit marker method.
However, in the 2-bit marker method, the existence of a common error bit on the marker leads to a decline in the error correcting capability. For example, if the first bit of the marker value “01” is a normal single-bit error, then the value “11” gets detected as the marker value.
Thus, the existence of a common error bit on the marker leads to inaccurate correction, which in turn triggers a shift error and thus causes a burst error.